wheel-build: Work around pip 19.0 errors

As pointed out by ajaeger; the wheel build jobs are failing with

 Error installing 'pip from
 git+https://github.com/ianw/pip.git@path-and-hash#egg=pip': editable
 mode is not supported for pyproject.toml-style projects.

This appears to be an issue with pip 19.0 which would be installed on
the build hosts, interacting with projects that have a pyproject.toml
file.

The problem only seems to be related to installing in editable mode,
which we don't really need here on this ephemeral host.  So just
switch to a regular install, with upgrade/reinstall to be sure.

[1] https://github.com/pypa/pip/pull/6449

Change-Id: I006500a2c98f512176fbac4ab426a8719243fc70

roles/build-wheels/files/wheel-build.sh

@@ -28,7 +28,8 @@ for BRANCH in master $BRANCHES; do
     # info into the logs to debug corrupt wheels.  We should see pip
     # stamping sha256 hashes into the logs for each wheel, so we can
     # see if the bad output is coming from pip, or somewhere else.
-    build_env/bin/pip install -e 'git+https://github.com/ianw/pip.git@path-and-hash#egg=pip'
+    build_env/bin/pip install --upgrade --force-reinstall \
+                      'git+https://github.com/ianw/pip.git@path-and-hash#egg=pip'
 
     # SHORT_BRANCH is just "master","newton","kilo" etc. because this
     # keeps the output log hierarchy much simpler.